Marginally. It depends somewhat on what you do. But windows uses about 40-50 base processes to run the system. Most are idle and doing nothing 99.9% of the time, but under some circumstances, it may be that you would find the system faster with two CPUs instead of one.

The one exception to this is if you run a program that is multithreaded - which means it knows how to do things (or more) at once, by itself. Unfortunately, many products today ARE NOT multithreaded. Some, like SQL server are, but office, at least through 2007 version is not, for example.

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If your Windows 2003 Exchange 2003 VM is CPU bound (ie. CPU is task manager stays above 50%), then adding an additional vCPU may help. If you don't see the additional processor in task manager after you boot after making the change, you will need to reinstall the Hyper-V Integration compnents.

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soffcecManagerAuthor Commented: 2010-11-24

When I try to install the integration services the computer asks me to reboot again and again and never finish the setup.

I could not uninstall. There was nothing to uninstall.
The installation never worked. I have tried this on 3 other Windows 2003 servers , with no luck. All these servers was running on they own Intel and Intel Xeon machine before I turned them into virtual PC.
I did put up new image of XP as virtual machine for testing and I had none problem installing integration services on it. I am going to run Repair on this virtual image. Maybe it will change the HAL

1 vCPU means that the VM OS can only process 1 thread at a time. So, if there are any high CPU usage tasks running in Exchange then there would be very little left over for the OS to use elsewhere.

If the guest OS is Windows Server 2003 with the latest service pack then you can run 2 vCPUs. This gives the OS the ability to dedicate one of those vCPUs to Exchange tasks or other needs while the second vCPU would be used to keep background tasks and the like running. This is a loose analogy without getting into the actual processing.

Having 2 vCPUs always makes things a lot more efficient for a multi-threaded OS such as Windows.